In conventional digital transmission systems, as shown in FIG. 1, it is common practice to interleave symbols prior to transmitting them in order to counter the effect of errors occurring in "packets". In a received data train, isolated errors are readily corrected by using error correcting codes encoded by an encoder 10 in the transmitter 11 (and decoded by a decoder 12 in the receiver 13), however such correction is not possible when errors occur in "packets" 14, i.e. in the form of a relatively high number of successive errors since, in such cases, the error correcting capacity of the code is exceeded. A known way of mitigating this drawback when the transmitted data is encoded by means of an error correcting code, is to interleave symbols (using an interleaver 15 in the transmitter 11) after encoding so as to prevent a packet of errors masking several entire words. Thus, by virtue of the interleaving, the symbols of any given word are not bunched together but are spread out over successive words as actually transmitted and are thus transmitted in small correctable bursts 17, such that a packet of errors cannot mask all of the symbols in the same original word (i.e. word as encoded prior to interleaving). Naturally, symbols which are transmitted in this way need to be deinterleaved by a deinterleaver 16 in the receiver 13 on reception, after which they are retransmitted in their proper places within each word. If only a few isolated symbols are erroneous, then the error correcting code can reconstitute the correct word. In FIG. 1, the transmitting antenna is referred to as 18 and the receiving antenna as 19.
A particularly simple known method of interleaving is constituted by matrix interleaving in which symbols are interleaved in accordance with a given law and on a repetitive basis. If each word contains N successive symbols, it may be agreed that the first word as effectively transmitted shall be made up by the sequence of the first symbols in the first N original words (i.e. after the error correcting code has been applied), with the second word actually transmitted being made up by the sequence constituted by the second symbols of said N first words, and the third word by the sequence of third symbols in these N first words, and so on.
This method of interleaving is effective in countering accidental packets of errors, i.e. errors which are not due to deliberate enemy jamming, however its repetitive nature makes it vulnerable to electronic warfare and thus totally ineffective in such circumstances.
In order to counter jamming of the kind that may be used in electronic warfare, one known technique consists in using pseudo-random interleaving. In this case, symbols are not interleaved repetitively, but they are interleaved in a pseudo-random manner so that without knowing the pseudo-random interleaving law applied, it is not possible to locate the various symbols of a given original word within the received message.
The quality of interleaving, whether pseudo-random or otherwise, is measured by the minimum distance Lm that may occur after interleaving between two symbols which were initially consecutive in the same code word. For a code capable of correcting t errors and an estimated jamming length Lc, it is necessary for Lm&gt;Lc/t.
One known method of pseudo-random interleaving consists in writing the symbols sequentially into a vector of size M, and then in reading the symbols sequentially from addresses given by Xn=(Xo+n.p)MOD M, where p is prime relative to M. The constants p and Xo are changed for each transmitted vector. For the best values of p that have been found, the separation between two symbols of the same word is half the separation obtained using matrix interleaving, for given delay and equal size of buffer memory. Another drawback of this known method is that it does not give rise to genuinely pseudo-random interleaving.
Another prior art proposal consists in a pseudo-random interleaver constituted by a convolutional interleaver followed by a permutator of length L. The permutations are performed by writing L symbols into a vector and then in reading them from a pseudo-random sequence of addresses. In comparison with a matrix interleaver and for equivalent interleaving (i.e. the same minimum distance between two symbols which were initially consecutive in the original train), this prior interleaver suffers from the drawback of requiring a buffer memory which is twice as large. In addition, this interleaver is not free from periodic behavior. If a particular symbol in a given word is considered, e.g. a frame-synchronizing word, then the position of this symbol in the channel binary train lies within a window of L bits: the method is thus relatively easily jammed by an enemy.
The invention seeks to remedy these drawbacks and, more generally, to obtain high quality interleaving (a large distance after interleaving between the various symbols of a given original word), which is genuinely pseudo-random, which uses buffer memories of reduced size, and which does not require too great a restitution time for comparable parameters.